The present invention relates to an intervertebral disk stabilizing implant and a method of stabilizing two adjacent vertebrae. More specifically, the present invention relates to rectangularly-shaped disk implants which are expanded in the middle portion and are used for spinal fusion.
Treatment of a herniated disk in the neck and in the lumbar region continues to be a challenging field of medicine. The classical treatment for a ruptured disk is diskectomy, i.e., removal of the disk from between the vertebrae. In this process, all or a portion of the intervertebral disk is removed, leaving a defect which may continue to bother the patients throughout the rest of their lives. An additional procedure is to replace the disk space with a bone graft, usually bone chips cut from the patient's iliac crest, bringing about fusion of the vertebrae above and below the disk, eliminating the empty space between the vertebrae.
No prosthetic disk has been shown to be safe and efficacious to date, so the best alternative treatment currently available is a simple and safe way to fuse the adjacent vertebrae. However, diskectomy with fusion is not ideal because the replaced bone does not have the function of the cartilaginous tissue of the disk, i.e. no cushioning effect, and has complications because of several factors. First, conventional bone plugs used to pack the disk space do not conform to the space of the disk because the disk bulges maximally in the center. The disk space is wider in the middle and narrower at its anterior and posterior ends. For this reason, the various bone plugs which are currently available commercially have only four contact points, i.e. at the front and back of the disk space. Secondly, access to the disk is from the side of the dorsal spine of the adjacent vertebrae, leaving a space that is "off-center" relative to the bodies of the adjacent vertebrae such that the stability of the implant is even more problematical than might be apparent from the limited contact resulting from the shape of the intervertebral space. Another complication is the possibility of infection or other conditions which may require the removal of the implant. Also, if the bone pieces do not fuse, they may eventually extrude out of the disk space, causing pressure on the nerve roots.
Various prosthetic disk plugs, or implants, are disclosed in the art, but all are characterized by limitations of not conforming to the shape of the disk space, lack of stability when inserted off-center, inability to be removed, or other disadvantages. For instance, U.S. Pat. No. 4,863,476 (and its European counterpart, EP-A-0260044) describes an elongated body divided longitudinally into two portions having a cam device movable therebetween for increasing the space between the two body portions once inserted into the disk space. However, that device is generally cylindrical in shape such that the only contact points between the device and the vertebral bodies are at the front and back of the disk space, creating increased likelihood of instability and generally rendering that device unsuitable for use after partial diskectomy. The art also discloses intervertebral disk prostheses (e.g., U.S. Pat. Nos. 3,867,728,4,309,777,4,863,477 and 4,932,969 and French Patent Application No. 8816184) which may have more general contact with the adjacent disks, but which are not intended for use in fusion of the disks. The art also includes spinal joint prostheses such as is described in U.S. Pat. No. 4,759,769, which is again not indicated for use when fusion is the preferred surgical intervention.
The problem of maintaining the spacing between vertebrae is particularly acute in the cervical vertebrae. The surgery itself is not as difficult as in the lumbar spine because access to the intervertebral space is from the front in the cervical spine, e.g., ventrally to the patient. Bone chips are not substantial enough to maintain the spacing between vertebrae, so the accepted surgical method to maintain spacing between adjacent vertebrae in the cervical spine is to scoop out the entire damaged disk, drill a hole into the intervertebral space, and insert a plug of the patient's bone into the space between vertebrae.
However, the cervical vertebrae are smaller than the vertebrae in the other portions of the spinal column such that there is little tolerance for anterior or posterior movement of the implant in the disk space such as may occur before the fusion is complete. Specifically, because of the relatively small tolerances, almost any movement posteriorally in the disk space imperils the nerves of the spinal cord. Bone plugs have no structure to resist such movement. Consequently, the hole into which the bone plug is inserted is slightly undersized relative to the plug so that the ligaments on either side of the spinal column help hold the plug in place by compressing the plug between vertebrae. Even so, the implant can move in the intervertebral space, so there is a need for a spinal implant for stabilizing adjacent vertebrae which maintains sufficient space between the vertebrae to allow the spinal nerves to pass between the processes of the cervical vertebrae without impingement by the vertebrae and which does not allow anterior or posterior movement of the implant in the disk space.
Similarly, the bone chips packed into the intervertebral space around the plug can be extruded out of the space posteriorally into contact with the spinal cord or the spinal nerves by the compression provided by the ligaments which help hold a bone plug in place. There is, therefore, a need for an implant, and a method of stabilizing adjacent vertebrae, which eliminates the possibility of impingement of the nerves of the spinal cord and/or the spinal nerves. In that same manner, there is a need for an implant and method for stabilizing more than two adjacent vertebrae of the spinal column without imperiling the spinal cord and spinal nerves and without placing screws into the spinal bodies.